An anarchist group claimed responsibility on Friday for kneecapping an Italian nuclear engineering executive and warned it would strike another seven times at the firm's parent company, Finmeccanica.

In a four-page letter sent to an Italian newspaper, the group, calling itself the Olga Nucleus of the Informal Anarchist Federation-International Revolutionary Front, said two of its members had shot Roberto Adinolfi, the CEO of Ansaldo Nucleare, in Genoa on Monday.

The firm is owned by Italian state-controlled defence and aerospace group Finmeccanica, which operates 16 sites and employs 10,000 people in the UK.

The letter, which was deemed credible by investigators, said the cell named itself after Olga Ikonomidou, one of eight Greek anarchists it listed as currently jailed in Greece. Seven further attacks would be carried out, one for each of them, the letter stated.

After the shooting Finmeccanica's CFO, Alessandro Pansa, said the firm would not be intimidated. On Friday a spokesman declined to comment on the letter.

The letter takes aim at Adinolfi, calling him a "sorcerer of the atomic industry" and criticising him for claiming in an interview that none of the deaths during the Japanese earthquake and tsunami in 2011 were due to nuclear incidents.

"Adinolfi knows well that it is only a matter of time before a European Fukushima kills on our continent," the letter stated.

"Science in centuries past promised us a golden age, but it is pushing us towards self destruction and slavery," the group wrote, adding: "With our action we give back to you a small part of the suffering that you scientists are bringing to the world."

Adinolfi, who was discharged from hospital under police guard on Friday after he was wounded in the shooting, said "Thank God I am OK".

Before the letter arrived at the offices of Corriere della Sera in Milan, investigators had suspected the attack could be the work of the Red Brigades, the terrorist organisation that kidnapped former Italian prime minister Aldo Moro in 1978.

The so-called Olga Nucleus stated that another cell within the Informal Anarchist Federation had sent a letter bomb to Italy's tax collection agency, Equitalia, in December, nearly blinding an official. Other letter bomb attacks in Italy have also been claimed by anarchist cells within the Federation.

As Italy's economy dips, Equitalia offices have become a target for violence. After an armed man briefly took hostages in an office in Bergamo last week, police on Friday clashed with protesters outside a Naples office, while a suspect package containing powder was sent to a Rome office.

In its letter, the Olga Nucleus said it could have chosen to attack Equitalia but was not looking to win public support. "We have nothing to do with citizens who are indignant about something which doesn't work in a system in which they want to be a part," it wrote.

"We are wild lovers of freedom, and will never renounce the revolution or the complete destruction of the state and its violence."